Reduction of wood to a fibrous state by mechanical attrition for the production of mechanical pulp has long been recognized as advantageous because of the high pulp yields approaching 100% as compared with chemical pulping methods in which the wood is cooked in a chemical reagent which reacts with or dissolves lignin to effect fiber separation.
The fibers in wood are bonded together by lignin and must be separated from one another to be useful in papermaking. As noted, chemical methods can do this by largely dissolving the lignin so as to free the fibers. However, mechanical methods do this by physically forcing the fibers apart so that the lignin is not removed for the most part. The oldest method of making mechanical pulp is by grinding wet logs, and such pulp is referred to as groundwood pulp. A more modern mechanical pulping method utilizes a disc refiner in which wet wood chips are mechanically defibered by rotating discs, and the resultant pulp is referred to as refiner mechanical pulp (RMP).
In the disc refiner, the lignin is somewhat softened by the frictional heat generated in the mechanical defibering process which facilitates fiber separation. The lignin is not softened enough to flow, however, and the fibers are not easily pulled out of their lignin ensheathment. This results in the fracture of some of the fibers in addition to the continued adhesion of some of the lignin to some of the fibers. The inherent strength of paper is due principally to hydrogen bonding between cellulose fibers, but lignin does not bond effectively to itself. Consequently, the strength of the paper formed from refiner mechanical pulp is generally inferior to that of paper produced from chemical pulp.
In recent years so-called thermomechanical pulp (TMP) has gained wide acceptance in the industry. In the thermomechanical pulping process wood chips are preheated with steam at an elevated temperature and pressure and are then defibered in a disc refiner also at an elevated temperature and pressure. As a result of the increased temperature due to the preheating step, the lignin is softened to a greater extent than in conventional mechanical pulping processes. Consequently, the cellulosic fibers are pulled apart and separated more easily to obtain comparatively intact whole fibers with substantially less fragmentation of the fibers. It would be expected that the whole fibers in thermomechanical pulp would result in stronger paper than can be obtained from groundwood pulp or from refiner mechanical pulp, but such improved strength has not been realized in most cases. Although lignin may become detached from the fibers to some extent during the mechanical defibering step, in general the heat softened lignin tends to redeposit on the fibers and harden in place as the pulp cools so that the fibers are lignin coated. This lignin coating of the fibers results in poor fiber-to-fiber bonding during subsequent paper manufacture.
Thus, even though thermomechanical pulp is attractive because of the high pulp yields and the diminished fiber fragmentation inherent in the thermomechanical pulping process, nevertheless, for many purposes the strength of the paper made from thermomechanical pulp is inadequate. In current practice, it is frequently necessary to upgrade thermomechanical pulp by blending with it a substantial amount, e.g. 10-25%, of a conventional chemical pulp such as kraft pulp in order to obtain the required strength in the paper, thereby increasing the overall cost of producing an acceptable paper.